1. Field of the Invention
The present invention relates generally to the field of nanotechnology and more particularly substrates for catalyzing the growth of carbon nanotubes, methods for preparing the substrates, and methods employing the substrates to grow carbon nanotubes.
2. Description of the Prior Art
A nanotube is a molecule composed of carbon atoms formed in the shape of hollow cylinder. The unique structural, mechanical, and electrical properties of nanotubes make them potentially useful for use in microscopic electrical, mechanical, and electromechanical devices. Nanotubes can be utilized individually or as an ensemble to build devices. For instance, individual nanotubes have been used as tips for scanning probe microscopy and as mechanical nano-tweezers. Ensembles of nanotubes have been used for field emission based flat-panel displays, and it has been suggested that bulk quantities of nanotubes may be used as a high-capacity hydrogen storage media.
Multi-walled nanotubes consist of multiple nanotubes of different diameters nested together, one inside another. For many applications, however, single-walled carbon nanotubes (SWNT) are desired. For example, SWNT-based miniature sensors have been shown to be sensitive gas sensors and selective biosensors. Additionally, individual semiconducting SWNTs have been made into field effect transistors.
Carbon nanotubes are typically produced by methods such as arc-discharge, laser ablation or chemical vapor deposition (CVD). The first two methods rely on evaporating carbon atoms from solid carbon sources at a very high temperature. These techniques are inherently disadvantageous because solid carbon vaporization via electric arc or laser apparatus is both costly and difficult to operate on commercial or industrial scales. The CVD process involves heating a catalyst material to a high temperature in a reactor and flowing a hydrocarbon gas through the reactor for a period of time. The key parameters in nanotube CVD growth include the hydrocarbon species in the gas, the catalysts, and the reaction temperature.
Typical catalysts for CVD nanotube growth are transition-metal nanoparticles, comprising elements such as iron, nickel, or cobalt, formed on a host material such as alumina. Notably, iron, cobalt and nickel are also the favored catalytic metals used in laser ablation and arc-discharge. For example, U.S. Pat. No. 5,500,200 discloses a method for the bulk production of multi-walled nanotubes using a catalyst prepared from iron acetylacetonate deposited on a host of fumed alumina particles with an average particle size of about 10 nm.
In another example, U.S. Pat. No. 6,346,189 discloses a CVD method that uses an island of a catalyst disposed on a substrate with a carbon nanotube extending form the island. Also, U.S. Pat. No. 6,333,016 discloses a method for producing carbon nanotubes by contacting a carbon containing gas with metallic catalytic particles, where the catalytic particles contain at least one metal from Group VIII and at least one metal from Group VIB of the Periodic Table of the Elements.
Additionally, U.S. Pat. No. 6,596,187 discloses a method of forming a nano-supported sponge catalyst on a substrate. In this method a catalytic metallic element and a structural metallic element are both deposited on the substrate to form a mixed metal alloy layer. The mixed metal alloy layer is then etched with an etchant to oxidize the catalytic metallic element and the structural metallic element and to remove at least a portion of the structural metallic element.
Ren et.al. disclose a method for producing nanotube arrays by first depositing a thin nickel layer onto mesoporous silica by radio frequency magnetron sputtering followed by plasma-enhanced hot filament CVD to synthesize the nanotubes. The nanotubes produced by this method, however, are generally multi-walled. See Science 282, 1105-1102 (1998), which is incorporated herein by reference.
Atomic force microscopy (AFM), a form of scanning probe microscopy, has been a powerful tool for a wide range of fundamental research and technological applications. A critical limitation of atomic force microscopy is the size and the shape of the scanning probe tip which dictate the lateral resolution and fidelity of AFM images. U.S. Pat. Nos. 6,346,189 and 6,401,526 disclose methods for providing AFM probe tips enhanced with carbon nanotubes.
The methods described above, however, do not provide a controlled method of producing a homogenous catalyst, nor do they provide a readily controllable yield of carbon nanotube growth. Therefore, what is needed are methods for producing homogenous catalysts and for readily controlling carbon nanotube growth.